Weisgerber (U.S. Pat. No. 2,961,366) teaches the use of polyvinylamine to improve the retention of ketene dimer by paper fibers, the increased retention resulting in a greater degree of sizing. In his teachings, Weisgerber indicated that the polyvinylamine could be added separately from the sizing agent to the pulp slurry, but the preferred mode of addition was to the aqueous emulsion of ketene dimer just prior to adding the ketene dimer to the papermaking system.
As Weisgerber taught the addition of polyvinylamine to the aqueous emulsion of ketene dimer just prior to adding the emulsion to the papermaking system, he was not concerned with the long term stability of the blend. However, for aqueous emulsions of alkyl ketene dimer to be commercially viable they must be stable for extended periods of time. Aqueous emulsions of ketene dimer must be both physically and chemically stable. Physical stability refers to sufficiently stable viscosity that the emulsions remain pumpable and dilutable until added to the papermaking system. Chemical stability refers to maintaining the assay of the ketene dimer in the emulsion at a high level until added to the papermaking system.
Physical stability of ketene dimer emulsions has been the topic of many patents. For example, Edwards, et al. (U.S. Pat. No. 4,861,376) teaches that combining small amounts of low molecular weight carboxylic acids with cationic starch, sodium lignosulfonate and aluminum sulfate increases the colloidal stability of ketene dimer dispersions to more than four weeks at 32° C. Schmid, et al. (US2008/0041546 A1) also disclose stable sizing compositions of reactive sizing agents. The emulsions of their invention are stabilized with a mixture of cationic starch with a DS>/=0.05, anionic dispersant and a linear nitrogen-containing polymer. Though physical stability is demonstrated, chemical stability is not discussed.
Stable starch-stabilized dispersions of ketene dimer are well known in the industry, see, for example, U.S. Pat. No. 4,861,376 to Edwards, et al. or U.S. Pat. No. 4,964,915 to Blixt, et al. Simple blends of these starch-stabilized dispersions of ketene dimer with commercial polyvinylamine result in physically unstable products that gel within minutes. Blends of starch-stabilized dispersions of ketene dimer and polyvinylamine, that has been pH adjusted as disclosed in US2008/0041546 A1, are also physically unstable, solidifying on storage (see Example 5 of US2008/0041546 A1).